Although large glass facades improve the architectural appearance of a building, a general disadvantage is overheating of the building caused by excessive solar transmittance of the facade. To solve this problem either air conditioning or solar shading, or a combination of the two, must be used. One of the primary motivations for the increased emphasis on solar shading has been governmental regulations (combined with a rise in energy prices) which require new buildings to be more energy efficient.
Solar shading devices can be placed either inside or outside the window or be integrated into the glazing unit.
Examples of indoor shading include Venetian blinds, Roman blinds, blinds, curtains, or internal shutters. Internal solar shading is efficient in terms of shading undesired light but thermally ineffective as the solar radiation is transformed into heat on the surface of the shading device. Thus, interior shading lead to a different distribution of heat inside the building while the total solar energy transmitted through the façade is unchanged compared to the façade without interior shading.
Examples of exterior shadings include Venetian blinds, solar screens, shutters and shading lamellas. Exterior shadings are efficient both in terms of reducing light and solar energy transmitted through the façade. The most distinct disadvantage of exterior shading is the need for maintenance and cleaning, and frequent mechanical failures of movable devices.
Examples of integrated solar shadings are solar shading foils, integrated Venetian blinds, tinted or coated glass, and integrated solar screens. The efficiency of integrated solar shadings depend on the actual type considered—typically the efficiencies of the best types are close to that of exterior shadings both in terms of reducing light and solar energy transmitted into the building.
Solar cells or photovoltaic cells may be mounted in combination with solar shadings to achieve both the advantages of shading and of the generation of electricity from converted solar energy by the photovoltaic effect.
U.S. Pat. No. 2,849,762 (“Combination of window and sun-proof screen”) describes a solar shading device where Venetian blinds are integrated between two sheets of transparent material for example between the glass panes of a double-pane window. Usually Venetian blinds are suspended beneath a mounting bar which may comprise a device for tilting of the slabs and raising or lowering the louvers inside the glazing. Proper functioning of the blind requires a free suspension of each of the louvers, thus the blind requires a glazing of minimum inner dimension as the width of each of the slabs, and it should be mounted in the centre of the glazing. This limits the use of Venetian blinds, as they cannot operate in glazings which are not mounted essentially vertically, i.e. roof glazings. Also, interior mounted Venetian blinds cannot be adapted to glazings with a different shape than rectangular or quadratic such as triangular glazings or glazings with a curved (arch) top. Interior mounted Venetian blinds require a mechanical or electrical feed through and this feed through is a generic weak spot in the sealing of the glazing.
Tinted or coated glass panes reduce the incident radiation irrespective of the angle of the solar light with the result that the windows might shield too much of the light during winter and too little during summer. Moreover, the tinted or coated glass panes might influence the perception of the colour of objects observed through the window and objects illuminated by the light passing through the window.
Another type of solar shading devices are semitransparent screens that consist of a non-transparent screen with a small perforations to allow for passage of light. U.S. Pat. No. 5,379,824 (“Double window apparatus”) describes an insect/solar screen which is placed between interior and exterior windows. The frame has air holes to allow for air flow into the space between the windows. A similar invention is presented in U.S. Pat. No. 6,315,356 (“Sunshade/Sunscreen combo”) where a semi-rigid planar sunscreen having a plurality of openings allows for passage of light and wind. In both these cases, the sunscreen is not integrated in the sealed cavity of an insulating glazing itself. Instead it can be considered an integral part of the window, and it can be used when the window is open.
Optical elements, in form of a semitransparent screens, that are integrated in insulated glazing units are normally suspended inside the cavity or fully laminated to the panes inside the insulated glazing unit. In U.S. Pat. No. 6,259,541 B1 (“Light dispersive insulated glazing unit”) a light dispersive film is integrated in the insulated glazing unit. The film is suspended midway between two glass panes and attached with an adhesive material at the edges of the window. This technique requires the glazing to be covered in full. Partial coverage is not possible as the film must be stretched between the spacer bars in order to maintain planarity. Furthermore, to avoid wrinkles the film is subjected to a heat treatment during the production, in which the film contracts to eliminate wrinkles.
A UV screen that is fully laminated to one of the interior sides of a double pane window is described in Patent US 200410209020A1. The space between the two glass panes is filled with an inert gas. This patent describes the method most commonly used in the glazing industry to integrate parts which are essentially flat plates or foils. The plate or foil is laminated between two sheets of PVB or EVA resin which again is laminated between two sheets of glass. The resins are cured under high pressure or vacuum while heated.
It remains a problem to provide an alternative method of integrating optical elements/solar screens in an insulated glazing unit.